Apparatus for Producing Films Stretched In-Line

ABSTRACT

The invention relates to an apparatus for producing tubular films stretched in-line by blow molding with an extruder, a film blowing line, a lay-flat unit, an oscillating haul-off unit, and a stretching line for monoaxially stretching the film laid flat in machine direction. According to the invention, the stretching line is arranged above the extruder between the lay-flat unit and the oscillating haul-off unit.

The present invention relates to an apparatus for producing tubular films stretched in-line by blow molding according to the generic part of claim 1.

For producing tubular films by blow molding, blown film extrusion lines usually are employed, which are in use already for a long time. To such lines plastics are supplied in granular form, which are then plastified in extruders under high pressures to obtain a viscous mass. This mass, which due to the pressure has a high temperature, is formed ring-shaped in a blowing head and escapes from the blowing head through a ring nozzle. Directly after leaving the ring nozzle, the mass already forms a film tube. This film tube can, however, be varied in its diameter, as it is not yet cooled completely. In general, the diameter is increased by blowing compressed air into the interior space of the film tube. To ensure that the film tube always has a constant diameter, it is guided at a distance to or directly along film guiding elements. In the field of blown film extrusion lines, this arrangement of the film guiding elements is referred to as calibration cage.

After passing through the calibration cage, the film tube can be laid flat in an oscillating haul-off unit, be installed and subsequently be stretched monoaxially in machine direction in a stretching line.

By stretching thermoplastic films, their properties can be varied selectively. Such properties for example are the transparency or the strength. Such stretching, which can be effected in transverse and/or longitudinal direction of the film web, can be performed in-line directly after the extrusion process. The process of elongating or stretching thermoplastic films is described for example in WO 2006/063641 A1 and in WO 2011/057918 A1.

In the production of tubular films, film thickness profile control systems with segmented control zones are used. By means of these systems, the film thickness profiles can be controlled such that the thickness deviations are as small as possible over the entire tube circumference.

From DE 39 411 85 A1, for example, a method for controlling the film thickness of tubular films from film blowing lines with downstream axial or biaxial stretching of the inflated tubular films in an oven is known, so that an end film is obtained which has as little deviations in thickness as possible.

During longitudinal stretching in a stretching line, the film is elongated in machine direction corresponding to the degree of stretching and the film thickness thereby is reduced. At the same time, the film necks in transverse direction, whereby the width of the film is reduced. This necking results in that the stretched film becomes thicker less and less from the film center in direction of the film edges, although it previously has been controlled to a thickness as constant as possible by blow molding. This increase in thickness is particularly pronounced at the film edge regions. During subsequent winding of the film, this causes a build-up of edges in film thickness. The film web is elongated more and more at the edges with increasing roll diameter.

To produce films with a thickness profile as uniform as possible, as they are required in particular for printing or laminating, the film edges which do not correspond to the desired thickness profile must be cut away. Due to this cutting away referred to as trimming, a large part of the film width however gets lost. Thus, on each side of the film up to about 200 mm are lost by trimming, i.e. cutting away.

From EP 2 277 681 B1 a generic method has become known, in which the film thickness profile of the tubular film produced in the film blowing line is controlled such that due to stretching a film with a thickness profile with deviations as small as possible from the mean film thickness across the entire film width is produced. For this purpose, a tubular film is produced by a film blowing process, which has two opposite thin spots. When the tubular film is laid flat, care now is taken that the thin spots are located in the film edge region and that the stretched film then has a thickness profile with deviations as small as possible from the mean film thickness. The thin spots incorporated selectively hence compensate the thickenings in the edge region obtained later on during stretching, so that a uniform film thickness extending up into the edge region should be obtained.

In the known apparatuses for producing tubular films stretched in-line by blow molding, the oscillating haul-off unit above the extruder usually is provided downstream of the lay-flat unit. The tubular film usually is passed on from the haul-off unit to the stretching line arranged beside the extruder and is stretched there correspondingly, before it is wound up on a winder. Such construction can be taken for example from EP 2 277 681 B1.

In this arrangement it is disadvantageous, however, that at the time at which it is introduced into the stretching line, the tubular film laid flat, installed and hauled off via the oscillating haul-off unit already has cooled to such an extent that for stretching it must still be heated to an operating temperature advantageous for stretching by means of at least one heating roller. This in particular has energetic disadvantages, as here the heating energy must be provided once again.

It therefore is the object of the invention to optimize a generic apparatus for producing tubular films stretched in-line by blow molding, in particular in energetic terms, i.e. to lower the energy demand for the production.

According to the invention, this object is solved by an apparatus with the combination of features of claim 1. For this purpose, an apparatus for producing tubular films stretched in-line by blow molding includes an extruder, a film blowing line, a lay-flat unit, an oscillating haul-off unit, and a stretching line for monoaxially stretching the film laid flat in machine direction. However, the stretching line is arranged above the extruder between the lay-flat unit and the oscillating haul-off unit. This means that the film originating from the extruder still is quite warm after being laid flat correspondingly in the lay-flat unit, until it is introduced into the stretching line. At least the first heating roller can be saved thereby. The film still warm can be stretched in the desired way by means of the stretching unit. For temperature control, however, further heating still is necessary. This heating, however, only serves for exactly adjusting the temperature, as the tubular film coming from the extruder and the lay-flat unit substantially still is close to the desired processing temperature.

Further advantageous aspects of the invention can be taken from the sub-claims following the main claim.

Advantageously, the stretching line together with the haul-off unit provided downstream in conveying direction is rotatably mounted on a platform above the lay-flat unit.

Advantageously, beside the stretching line further components, such as electrical converters and/or electric feed components and/or drives, also are arranged on the platform. The arrangement is chosen such that here a weight compensation is effected on the platform.

For the central supply of water and electricity above the platform, slip rings and a rotary water lead-through can be provided. Advantageously, the communication cables also can be guided onto the platform as data line.

Above the platform, observation systems in the form of cameras advantageously are arranged at various positions. Here, a respective ascent to the platform by the operating personnel can be omitted.

The rollers of the stretching line advantageously are driven and temperature-controlled individually. The rollers form at least one stretching nip, wherein the same advantageously is adjustable. The rollers of the stretching line can have different diameters, wherein the stretching rollers have a comparatively smaller diameter.

Advantageously, the rollers of the stretching line can have a non-stick coating.

Particularly advantageously, the rollers of the stretching line can be designed such that by the advance of at least one roller a stretching force of 100 N to 1000 N can be applied.

For temperature control of the rollers of the stretching unit, at least two temperature control circuits, namely a heating circuit and a cooling circuit, can be present. By means of the temperature-controlled rollers, the blown film guided through the stretching unit can be heated up to a temperature which lies at least 10° C. below the glass temperature of the tubular film material. According to a further preferred aspect it is essential that the temperature-controlled rollers have a temperature uniformity of <5° C., preferably <1° C., across the roller width.

Particularly advantageously, nip rollers also are provided in the stretching unit, which preferably have a surface quality other than the nipped rollers.

For saving energy, intermediate circuit couplings each can be provided between the driving and braking rollers in the stretching unit, which preferably are formed as electric couplings.

Instead of the reversing device downstream of the stretching unit, it might also be provided that the same is formed in the stretching device itself in that the annealing rollers in the stretching device are formed to be reversing at the upper region of the stretching device.

Further features, details and advantages of the invention will be explained in detail with reference to an exemplary embodiment illustrated in the drawing.

The only FIGURE shows a film blowing line with downstream stretching unit according to an embodiment of the present invention.

The FIGURE shows an apparatus 10 according to the invention for producing tubular films stretched in-line by blow molding. Via a dosing device, the plastic granules to be processed are supplied to an extruder 12, molten in the same, homogenized and supplied to a film blowing head 14. The film blowing head 14 includes a ring nozzle from which the extruded plastic mass exits. Through the film blowing head cooling air is supplied for inflating a tubular film 16. During the blowing operation, the tubular film 16 is kept in shape by a calibration head 18.

After solidification of the plastic material, the tubular film 16 is laid flat in a lay-flat unit 20 and after the lay-flat unit supplied directly to a stretching unit 22 arranged above the film blowing head 14. The stretching unit is arranged on a platform 24 which lies above the lay-flat unit 20. Here, the still warm tubular film 16 laid flat as supplied from the lay-flat unit 20 is stretched monoaxially in machine direction. The correspondingly stretched film then is hauled off continuously via a haul-off unit 24 arranged above the stretching unit 22 and supplied to a winder 28 via various rollers 26 and wound up to form film rolls.

In the stretching unit 22 a stretching force of about 100 N to 1000 N is applied due to the advance of a roller. The rollers are at least partly heated, in order to smooth the tubular film 16 during stretching.

In the stretching unit 22 shown here, four stretching nips are shown. Usually, a corresponding stretching unit must have at least one stretching nip. In the design variant shown here, all rollers of the stretching unit 22 are driven and temperature-controlled individually. As indicated here in FIG. 1, the rollers each have a different diameter, wherein the stretching rollers have a comparatively smaller diameter. In a manner not shown here, nip rollers, i.e. counter rollers, also can be provided in the stretching unit 22.

Advantageously, the rollers in the stretching unit 22 have a non-stick coating. The respective stretching nips likewise can be adjustable advantageously. Usually, a temperature of <10° C. below the glass temperature is adjusted via the heating rollers. In one design variant, four rollers without haul-off nip with a wrap angle of more than 60° or preferably even more than 90° can be provided in the stretching unit.

In the exemplary embodiment according to FIG. 1 as shown here, the stretching unit 22 is set up vertically. According to an alternative design variant, which is not shown here, it can also be mounted transversely on the platform 24.

In accordance with the invention, the torque of the rollers in the stretching unit is adjusted such that a certain controlled web tension factor is set. In a manner not shown here, one knife or several knives are present above the stretching unit, which perform the cut for the so-called trimming. Here, the edge regions which due to stretching are formed comparatively thicker than the middle regions are cut away. However, to minimize the loss caused by cutting off the thickened edge regions, a profile control for a more uniform film thickness across the entire region of the film thickness advantageously is provided in the apparatus for producing tubular films stretched in-line as shown in FIG. 1, as it has already been described in detail for example in the German Patent Application 10 2013 007 669. For carrying out such profile control, a profile measurement at the correspondingly necessary points, as they have been described for example in DE 10 2013 007 669, also is to be provided in a manner not shown in detail in FIG. 1. 

1. An apparatus for producing tubular films stretched inline by blow molding with an extruder, a film blowing line a lay-flat unit, an oscillating haul-off unit, and a stretching line for rnonoaxially stretching the film laid flat in machine direction, wherein the stretching line is arranged above the extruder between the lay-flat unit and the oscillating haul-off unit.
 2. The apparatus according to claim 1, wherein the stretching line together with the haul-off unit provided downstream of the same in conveying direction is rotatably mounted on a platform above the lay-flat unit.
 3. The apparatus according to claim 2, wherein on the platform beside the stretching line further components such as electrical converters and/or electric feed components and/or drives are arranged.
 4. The apparatus according to claim 2, wherein for the central supply of electricity and water slip rings and a rotary water lead-through are provided above the platform and/or a communication cable is guided onto the platform as data line.
 5. The apparatus according to claim 1, wherein above the platform observation systems, preferably cameras, are arranged at various positions.
 6. The apparatus according to claim 1, wherein the rollers of the stretching line are driven and temperature-controlled individually and the rollers form at least one stretching nip, wherein the same advantageously is adjustable.
 7. The apparatus according to claim 1, wherein the rollers of the stretching line have different diameters, and the stretching rollers have a comparatively smaller diameter.
 8. The apparatus according to claim 1, wherein the rollers of the stretching line have a non-stick coating.
 9. The apparatus according to claim 1, wherein the rollers of the stretching line are designed such that by the advance of at least one roller, a stretching force of 100 N to 1000 N can be applied.
 10. The apparatus according to claim 1, wherein for tempering the rollers of the stretching unit, at least two temperature control circuits, namely a heating circuit and a cooling circuit, are present.
 11. The apparatus according to claim 1, wherein the temperature-controlled rollers, the blown film guided through the stretching unit is heated up to a temperature which lies at least 10° C. below the glass temperature of the tubular film material.
 12. The apparatus according to claim 1, wherein the temperature-controlled rollers have a temperature uniformity of <5° C., preferably <1° C., across the roller width.
 13. The apparatus according to claim 1, wherein nip rollers are provided in the stretching unit, which preferably have a surface quality other than the nipped rollers.
 14. The apparatus according to claim 1, wherein between driving and braking rollers in the stretching unit an intermediate circuit coupling, preferably an electric coupling, is provided for saving energy.
 15. The apparatus according to claim 1, wherein instead of the reversing device downstream of the stretching unit, the annealing rollers of the stretching device are formed to be reversing.
 16. The apparatus according to claim 3, wherein for the central supply of electricity and water slip rings and a rotary water lead-through are provided above the platform and/or a communication cable is guided onto the platform as data line.
 17. The apparatus according to claim 16, wherein above the platform observation systems, preferably cameras, are arranged at various positions.
 18. The apparatus according to claim 4, wherein above the platform observation systems, preferably cameras, are arranged at various positions.
 19. The apparatus according to claim 3, wherein above the platform observation systems, preferably cameras, are arranged at various positions.
 20. The apparatus according to claim 2, wherein above the platform observation systems, preferably cameras, are arranged at various positions. 